1. Field of the Invention
The present invention relates to a method for producing an epitaxial silicon single crystal wafer, and an epitaxial silicon single crystal wafer having low concentration of heavy metal impurity and excellent crystallinity.
2. Description of the Related Art
An epitaxial silicon single crystal wafer has been used since a long time ago as a wafer for producing a discrete semiconductor or a bipolar IC. It has also been used for MOS LSI since it hardly causes software error and has excellent latch up property, and therefore has been used for a microprocessor unit or as a flash memory device. Furthermore, in order to reduce reliability failure of DRAM due to grown-in defects, needs for the epitaxial silicon single crystal wafer is gradually increasing.
When heavy metal impurity is present in the epitaxial silicon single crystal wafer used for a semiconductor device, characteristics of the semiconductor device are degraded. Particularly, cleanliness which is required for an advanced device is considered to be a concentration of heavy metal impurity of 1.times.10.sup.9 atoms/cm.sup.2 or less. Accordingly, heavy metal impurity has to be reduced as much as possible.
There has been increased importance of gettering technology which is one of techniques for reducing such a heavy metal impurity. Conventionally, a substrate having high boron concentration (for example, 3.times.10.sup.18 atoms/cm.sup.3 or more, 0.02.OMEGA..multidot.cm or less) which is excellent in gettering effect has been used for producing an epitaxial silicon single crystal wafer. Accordingly, high device yield has been achieved in the epitaxial silicon single crystal wafer compared to the silicon single crystal wafer having general (ordinary or medium) resistivity produced by Czochralski method.
However, along with a recent tendency to use a substrate having lower boron concentration than that of the conventional substrate as an epitaxial silicon single crystal wafer for CMOS device, there is caused a problem of lower gettering capability compared to a substrate having high boron concentration. Moreover, the problem of low gettering capability is caused even in the substrate having high boron concentration depending on the concentration.
In the case of a high boron concentration substrate having a resistivity less than 0.1.OMEGA..multidot.cm, there are problems that there is easily caused auto-doping wherein impurity vaporized from the substrate during epitaxial growth is incorporated again into the epitaxial layer and impurity is incorporated from the surface of the substrate to the epitaxial layer by solid phase out-diffusion. Since the resistivity of the epitaxial layer may be changed as a result of auto-doping, it is necessary to take measures such as CVD coating of the substrate with SiO.sub.2, which may lead to low productivity and high production cost.